The present invention relates to a probe head for a coordinate measuring machine for determining spatial coordinates on a measurement object, and to a probe tool for such a probe head.
Coordinate measuring machines are typically used for determining geometrical dimensions and/or the shape of workpieces. In general, the coordinate measuring machine has a measurement head which can be moved relative to the workpiece (hereafter measurement object). Depending on the position of the measurement head relative to the workpiece, spatial coordinates are determined which represent the position of defined measurement points on the workpiece within a measurement volume. If a plurality of spatial coordinates are determined for a plurality of measurement points, the geometrical dimensions and/or the shape of the measurement object can be checked with the aid of the spatial coordinates.
In many cases, the probe head of a coordinate measuring machine has a probe tool with which the desired measurement points on the measurement object are touched (physical contact). Accordingly, the probe head may be referred to as a touch probe head.
The probe tool often has a stylus with a free end, on which a touch ball is arranged. The desired measurement points on the measurement object are touched with the touch ball. In order to reach all measurement points in the case of complex workpieces, it is often desirable to hold the stylus in different orientations relative to the probe head. Furthermore, it is often desirable to use probe tools with different styli or styli combinations on a probe head, in order to optimally reach all desired measurement points.
DE 101 14 126 A1 discloses a probe head having a replaceable probe tool. The probe tool has a stylus which is angled in an L-shape and can be fastened on the probe head in different rotation angle positions. A rotary plate of the probe tool is arranged in a defined rotation angle position on the probe head by means of a kinematic three-point bearing. In order to change the rotation angle position, the rotary plate is disengaged from the three-point bearing by means of a pneumatically actuated push-rod and subsequently rotated on the push-rod by means of a rotary drive arranged in the probe head. The rotary plate is then re-engaged into the defined three-point bearing. The rotation angle position of the rotary plate and of the stylus arranged thereon is determined by a sensor, which at the same time also delivers signals for the rotary drive. In order to prevent the rotary plate from falling when the stylus is being rotated, the rotary plate is fastened on the push-rod by a hook-shaped connection. Further safeguarding of the rotary plate against falling during the rotation process is not disclosed.
The known probe head makes it possible to use different probe tools with different orientations. It is therefore suitable for flexible measurements on complex measurement objects. Disadvantages are the rotary drive integrated in the probe head and the pneumatically actuated push-rod, which are required in addition to the probe head sensors with which displacements of the stylus relative to the probe head are determined. Depending on the frequency of use, the rotary drive generates heat which can have an unfavorable effect on the measurement accuracy of the probe head. Furthermore, integration of the additional components into the probe head leads to heavier weight.
In U.S. patent application Ser. No. 13/198,845, published as US 2012/0079731 A1, the assignee has described a probe head for a coordinate measuring machine, in which the probe tool can be rotated using measurement force generators and a so-called roll motion projection. Measurement force generators are actually used in so-called active probe heads to adjust and ensure a defined sampling force when a measurement point is being touched. They are present in active probe heads, and according to the concept described in US 2012/0079731 A1, they are also used for rotating the probe tool. Accordingly, this probe head avoids the disadvantages of the probe head of DE 101 14 126 A1.
In the case of the probe head of US 2012/0079731 A1, the rotary plate of the probe tool is fastened on a retaining pin, by means of which the rotary plate can be disengaged from its kinematic bearing for the rotational movement. In test operation, it has been found that the fastening of the rotary plate on the retaining pin is a critical point. If the retaining pin and/or the rotary plate are contaminated, for example, it is possible that the fastening of the rotary plate on the retaining pin will be insufficient and the probe tool can loosen and fall during rotation on the roll motion projection. This may cause damage to the probe tool and/or the measurement object.
DE 10 2005 043 454 B3 discloses a change device for replaceable reception of a probe tool on a coordinate measuring machine, a safety cable ensuring that the probe tool does not fall out of the holder even in the event of collision, even though the probe tool can detach from the holder in the event of a collision in order to avoid damage. A similar safety mechanism is also described in the aforementioned prior application US 2012/0079731 A1. This safety mechanism, however, does not offer protection against the probe tool falling out when the fastening of the probe tool on the retaining pin is already insufficient.
EP 0 523 906 A1 discloses a further probe head having a rotatable probe tool. The probe tool is in this case retained magnetically on the probe head. A retaining magnet for holding the probe tool is connected to a shaft which can be rotated by means of a motor specially provided therefor. In order to rotate the probe head, the retaining magnet is moved forward together with the shaft, so that the probe tool is disengaged from its working position. With the aid of the motor, the probe tool is then rotated. Subsequently, the shaft is pulled back into its original position, the retaining magnet being released from the probe tool. Here again, no particular security is provided against the probe tool falling during rotation.